1. Field of the Invention
This invention relates to a silicon wafer possessing an excellent gettering ability and a method for the production thereof. More particularly, it relates to a silicon wafer of excellent gettering ability for use in electronic devices such as integrated circuits and a method for the production thereof.
2. Description of the Prior Art
In the production of integrated circuit devices on silicon wafers, when defects, defiling substances, or impurities are present in or are introduced into the surface part of silicon wafers on which the devices are formed, they induce the phenomenon of immoderate current leakage and seriously affect the yield of acceptable devices from the production line. These harmful defects, defiling substances, and impurities can be relocated to a certain extent from device-forming regions to harmless regions in substratal material. This fact has found recognition in the technical field. In the technical field of the electronics industry, the method and process for dispersing and trapping such defects, defiling substances, and impurities occurring, in active device regions prior to and during the formation of devices are referred to as "gettering.".
The method which, for the impartation of a gettering ability to a silicon wafer, comprises subjecting silane gas to thermal decomposition in the presence of the silicon wafer, thereby causing the vapor-phase growth of a polycrystalline silicon layer on the rear surface of the silicon wafer, has been known to the art [Japanese Patent Laid-Opens SHO 58(1983)-138,035, SHO 59(1984)-186,331, and SHO 52(1977)-120,777].
Incidentally, the gettering with the polycrystalline silicon layer is accomplished by the fact that the impurities present in the crystal silicon wafer are trapped in the grain boundaries of polycrystalline silicon. For the enhancement of the gettering ability, it is necessary that the polysilicon layer should be formed in intimate contact with the substrate of single crystal silicon and, moreover, the polysilicon layer should possess a large grain boundary surface area. For the grain boundary surface area to be amply large, the individual crystal grains are desired to be small and uniform.
In the case of the silicon wafer disclosed in Japanese patent Laid-Open- SHO 52(1977)-120,777, it is demonstrated in Example 1 that when one lateral half of the oxide film 2,700 .ANG. in thickness formed on the rear side of the silicon wafer was etched off, a 1.6 microns poly silicon film was superposed on the rear surface of the silicon wafer, and the silicon wafer thus treated was evaluated for the effectiveness of the gettering process due to the decrease in leak currents of MOS capacitors, the leak current on the side having the oxide film etched off was decreased approximately by two orders of magnitude. Example 1 clearly purports to express that the absence of an oxide film between the silicon substrate and the polysilicon film is an essential requirement for the impartation of the gettering effectiveness.
We have repeated the example as follows. A sample silicon wafer was immersed in 1% hydrofluoric acid to etch off the oxide film on the surface of silicon wafer and expose the underlying surfaces of single crystal silicon. For the wafer to be prevented from growing an oxide film on the surface thereof, it was placed in a low pressure chemical vapor deposition (LPCVD) oven swept in advance at room temperature with nitrogen gas passed at a flow rate of 20 liters/min. Then, the interior of the oven holding the wafer was vacuumized to 40 Pa and heated from room temperature to 650.degree. C. under continued flow of nitrogen gas supplied at a rate of 0.5 liter/min. When the temperature reached 650.degree. C., silane gas was supplied to the oven at a rate of 0.35 liter/min as carried by a flow of nitrogen gas fed at a rate of 0.5 liter/min for 120 minutes to deposit a polysilicon film on the surface of the silicon wafer. After the treatment of deposition, the surface of the silicon wafer was examined under an optical microscope and a scanning electron microscope. This examination revealed that absolutely no or only partial growth of polysilicon film occurred on the surface of the silicon wafer. On closer examination of the surface suffering from partial growth of the polysilicon film, it was found that particles produced within the vacuumized oven were adhering to the affected part or a polysilicon film was growing on the part to which particles in the liquid adhered when the oxide film was etched off with hydrofluoric acid. This observation may be logically explained by a postulate that on the surface of a silicon wafer from which an oxide film is etched off to expose the underlying surface of single crystal silicon, no growth of polysilicon film is allowed because the CVD reaction is epitaxial in nature, while only in the portion covered with particles, the polysilicone film grows in the form of an island.
By the method disclosed in Japanese Patent Laid-Open SHO 58(1983)-138,035, no sufficient gettering ability is obtained because the polysilicon layer is formed directly on the surface of the substrate of single crystal silicon.
By the method of Japanese Patent Laid-Open SHO 59(1984)-186,331, the polysilicon layer formed on the surface of the substrate of single crystal silicon is doped with oxygen. This doping with oxygen, however, rather impairs the gettering effectiveness to be brought about by the polysilicon layer.
An object of this invention, therefore, is to provide a silicon wafer possessing an excellent gettering ability and a method for the production thereof.